Binding Percona Distribution for PostgreSQL components to Specific Kubernetes/OpenShift Nodes

The operator does good job automatically assigning new Pods to nodes with sufficient resources to achieve balanced distribution across the cluster. Still there are situations when it is worth to ensure that pods will land on specific nodes: for example, to get speed advantages of the SSD equipped machine, or to reduce network costs choosing nodes in a same availability zone.

Appropriate sections of the deploy/cr.yaml file (such as proxy.pgBouncer) contain keys which can be used to do this, depending on what is the best for a particular situation.

Affinity and anti-affinity

Affinity makes Pod eligible (or not eligible - so called “anti-affinity”) to be scheduled on the node which already has Pods with specific labels, or has specific labels itself (so called “Node affinity”). Particularly, Pod anti-affinity is good to reduce costs making sure several Pods with intensive data exchange will occupy the same availability zone or even the same node - or, on the contrary, to make them land on different nodes or even different availability zones for the high availability and balancing purposes. Node affinity is useful to assign PostgreSQL instances to specific Kubernetes Nodes (ones with specific hardware, zone, etc.).

Pod anti-affinity is controlled by the affinity.podAntiAffinity subsection, which can be put into proxy.pgBouncer and backups.pgbackrest.repoHost sections of the deploy/cr.yaml configuration file.

podAntiAffinity allows you to use standard Kubernetes affinity constraints of any complexity:

affinity:
  podAntiAffinity:
    preferredDuringSchedulingIgnoredDuringExecution:
    - weight: 1
      podAffinityTerm:
        labelSelector:
          matchLabels:
            postgres-operator.crunchydata.com/cluster: keycloakdb
            postgres-operator.crunchydata.com/role: pgbouncer
        topologyKey: kubernetes.io/hostname

You can see the explanation of these affinity options in Kubernetes documentation.

Topology Spread Constraints

Topology Spread Constraints allow you to control how Pods are distributed across the cluster based on regions, zones, nodes, and other topology specifics. This can be useful for both high availability and resource efficiency.

Pod topology spread constraints are controlled by the topologySpreadConstraints subsection, which can be put into proxy.pgBouncer and backups.pgbackrest.repoHost sections of the deploy/cr.yaml configuration file as follows:

topologySpreadConstraints:
  - maxSkew: 1
    topologyKey: my-node-label
    whenUnsatisfiable: DoNotSchedule
    labelSelector:
      matchLabels:
        postgres-operator.crunchydata.com/instance-set: instance1

You can see the explanation of these affinity options in Kubernetes documentation.

Tolerations

Tolerations allow Pods having them to be able to land onto nodes with matching taints. Toleration is expressed as a key with and operator, which is either exists or equal (the latter variant also requires a value the key is equal to). Moreover, toleration should have a specified effect, which may be a self-explanatory NoSchedule, less strict PreferNoSchedule, or NoExecute. The last variant means that if a taint with NoExecute is assigned to node, then any Pod not tolerating this taint will be removed from the node, immediately or after the tolerationSeconds interval, like in the following example.

You can use instances.tolerations and backups.pgbackrest.jobs.tolerations subsections in the deploy/cr.yaml configuration file as follows:

tolerations:
- effect: NoSchedule
  key: role
  operator: Equal
  value: connection-poolers

The Kubernetes Taints and Toleratins contains more examples on this topic.

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Last update: 2023-05-22